Polysilicon design for replacement gate technology

ABSTRACT

The present disclosure provides an integrated circuit. The integrated circuit includes a semiconductor substrate; and a passive polysilicon device disposed over the semiconductor substrate. The passive polysilicon device further includes a polysilicon feature; and a plurality of electrodes embedded in the polysilicon feature.

BACKGROUND

Precise polysilicon resistor has been used in conventional integratedcircuit (IC) design. When a semiconductor device such as ametal-oxide-semiconductor field-effect transistor (MOSFET) is scaleddown through various technology nodes, a high k dielectric material andmetal are adopted to form a gate stack. For gate replacement process,the resistance of the formed polysilicon resistors is too low. A singlecrystalline silicon resistor has been proposed to resolve the issue.However, the single crystalline silicon resistor cannot provide preciseimpedance matching for the applications, such as analog, radio frequency(RF) and mixed-mode circuits.

SUMMARY

The present disclosure provides an integrated circuit. The integratedcircuit includes a semiconductor substrate; and a passive polysilicondevice disposed over the semiconductor substrate. The passivepolysilicon device further includes a polysilicon feature; and aplurality of electrodes embedded in the polysilicon feature.

The present disclosure also provides another embodiment of a method ofmaking an integrated circuit. The method includes forming a high kdielectric material layer on a semiconductor substrate; forming apolysilicon layer on the high k dielectric material layer; patterningthe polysilicon layer to form a polysilicon feature for a passivedevice; forming a plurality of electrodes embedded in the polysiliconfeature.

The present disclosure also provides another embodiment of a method ofmaking an integrated circuit. The method includes forming a high kdielectric material layer on a semiconductor substrate; forming apolysilicon layer on the high k dielectric material layer; patterningthe polysilicon layer to form a first dummy gate for a firstfield-effect transistor (FET), a second dummy gate for a second FET, anda polysilicon feature for a passive device; forming an inter-leveldielectric (ILD) layer on the semiconductor substrate; performing afirst chemical mechanical polishing (CMP) process to etch back the ILDlayer; removing a portion of the polysilicon feature from the passivedevice and the first dummy gate from the first FET, resulting in aplurality of column trenches in the passive device and a first gatetrench in the first FET; forming a first metal layer in the columntrenches and the first gate trench, wherein the first metal layer has afirst work function; forming a first conductive layer on the first metallayer; and performing a second CMP process to remove excessive portionof the first metal layer and the first conductive layer above a topsurface of the ILD layer.

The present disclosure also provides another embodiment of a method ofmaking an integrated circuit. The method includes forming a high kdielectric material layer on a semiconductor substrate; forming apolysilicon layer on the high k dielectric material layer; patterningthe polysilicon layer to form a first gate for a first field-effecttransistor (FET), a second gate for a second FET, and a polysiliconfeature for a passive device; forming, on the semiconductor substrate, apatterned mask having a first set of openings defining contact regionson the polysilicon feature of the passive device, and a second set ofopenings defining source/drain regions in the first FET; and applying afirst ion implantation to the semiconductor substrate to form electrodesembedded in the polysilicon feature within the first set of openings;and source and drain of the first FET within the second set of openings.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a flowchart of a method making a semiconductor device havingmetal gate stacks and polysilicon structures constructed according tovarious aspects of the present disclosure.

FIGS. 2-6 and 8-11 are sectional views of one embodiment of asemiconductor structure having metal gate stacks and polysiliconstructure at various fabrication stages constructed according to variousaspects of the present disclosure.

FIG. 7 is a top view of one embodiment of a polysilicon feature andelectrodes embedded in the polysilicon feature constructed according tovarious aspects of the present disclosure.

FIG. 12 is a top view of one embodiment of a semiconductor structurehaving a plurality of polysilicon stacks constructed according tovarious aspects of the present disclosure.

FIG. 13 is a flowchart of a method making a semiconductor device havingmetal gate stacks and polysilicon structures constructed according tovarious aspects of the present disclosure.

FIG. 14 is sectional view of one embodiment of a semiconductor structurehaving metal gate stacks and a polysilicon structure constructedaccording to various aspects of the present disclosure.

FIGS. 15-16 are sectional views of the polysilicon structure of FIG. 14in various embodiments constructed according to various aspects of thepresent disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a first feature over or on a second featurein the description that follows may include embodiments in which thefirst and second features are formed in direct contact, and may alsoinclude embodiments in which additional features may be formedinterposing the first and second features, such that the first andsecond features may not be in direct contact.

FIG. 1 is a flowchart of an embodiment of a method 100 making asemiconductor device having a metal gate stack constructed according tovarious aspects of the present disclosure. FIGS. 2 through 11 aresectional views of a semiconductor structure 200 at various fabricationstages and constructed according to one or more embodiments. Thesemiconductor structure 200 and the method 100 of making the same arecollectively described with reference to FIGS. 1 through 11.

The method 100 begins at step 102 by providing a semiconductor substrate210. The semiconductor substrate 210 includes silicon. Alternatively,the substrate includes germanium, silicon germanium or other propersemiconductor materials. The semiconductor substrate 210 also includesvarious isolation features such as shallow trench isolation (STI) formedin the substrate to separate various devices. The semiconductorsubstrate also includes various doped regions such as n-well andp-wells. In one embodiment, the semiconductor substrate 210 includes afirst region 212 and a second region 214. The first region 212 includesa trench isolation feature 216 and the second region 214 includesanother trench isolation feature 217. The formation of the STI mayinclude etching a trench in a substrate and filling the trench byinsulator materials such as silicon oxide, silicon nitride, or siliconoxynitride. The filled trench may have a multi-layer structure such as athermal oxide liner layer with silicon nitride filling the trench. Inone embodiment, the STI structure may be created using a processsequence such as: growing a pad oxide, forming a low pressure chemicalvapor deposition (LPCVD) nitride layer, patterning an STI opening usingphotoresist and masking, etching a trench in the substrate, optionallygrowing a thermal oxide trench liner to improve the trench interface,filling the trench with CVD oxide, using chemical mechanicalplanarization (CMP) to etch back, and using nitride stripping to leavethe STI structure. In another embodiment, the semiconductor substrate210 in the second region 214 includes a p-well 218 and an n-well 219.

The method 100 proceeds to step 104 by forming a polysilicon resistorstack (or resistor stack) 220 in the first region 212 and gate stacks222/224 in the second region 214. The resistor stack 220 and the gatestacks 222/224 are formed in a same processing procedure. The resistorstack 220 includes a high-k (high dielectric constant) dielectric layer226 formed on the STI feature 216. The high-k dielectric layer 226includes a dielectric material having the dielectric constant higherthan that of thermal silicon oxide, about 3.9. In one example, thehigh-k dielectric layer 226 includes hafnium oxide (HfO). In variousexamples, the high-k dielectric layer 226 includes metal oxide, metalnitride, or combinations thereof. In one example, the high-k dielectriclayer 226 includes a thickness ranging between about 10 angstrom andabout 20 angstrom. In one embodiment, the resistor stack 220 furtherincludes a conductive layer 228 disposed on the high-k dielectric layer226. In one example, the conductive layer 228 includes titanium nitride(TiN). In another example, the thickness of the titanium nitride layerranges between about 10 angstrom and about 30 angstrom.

The polysilicon resistor stack 220 further includes a polysilicon layer230 disposed on the conductive layer 228. The polysilicon layer isnon-doped to have a higher resistance or doped to have a properresistance. In one example, the polysilicon is doped to have aresistance higher than about 500 Ohm per square. Similarly, the gatestacks 222/224 also include the high k dielectric layer 226 disposed onthe substrate 210 in the second region 214. In one embodiment, the gatestacks 222/224 also include the conductive layer 228 disposed on thehigh-k dielectric layer 226. The gate stacks 222 and 224 also includethe polysilicon layer 230 disposed on the conductive layer 228. Forclarity, the portions of the polysilicon layer 230 in the resistor stack220 and the gate stacks 222/224 are referred to as 230 a, 230 b and 230c, respectively. In one embodiment, spacers 232 are disposed on thesidewalls of the polysilicon resistor stack 220 and the sidewalls of thegate stacks 222/224. Alternatively, an interfacial layer (IL), such assilicon oxide, may be interposed between the high-k dielectric layer 226and the substrate 210. Particularly, the interfacial layer is interposedbetween the STI feature 216 and the high-k dielectric layer 226 in thefirst region 212 and is interposed between the substrate 210 and thehigh-k dielectric layer 226 in the second region.

In one embodiment, the resistor stack 220 is formed for a passivedevice. This passive device can be used for a resistor or alternativelyused as a polysilicon fuse. In another embodiment, the first gate stack222 is formed in the second region 214 for an n-type field-effecttransistor (nFET) 234 and the second gate stack 224 is formed in thesecond region for a p-type field-effect transistor (pFET) 236. In oneembodiment, the nFET 234 is formed in the p-well 218. In anotherembodiment, the pFET 236 is formed in the n-well 219. The nFET 234further includes various doped regions, such as source/drain 238.Similarly, the pFET 236 further includes various doped regions, such assource/drain 240. In one embodiment, the nFET and pFET includemetal-oxide-semiconductor FETs (MOSFET) such as nMOSFET and pMOSFET.

In one embodiment of the forming the resistor stack 220 and the gatestacks 222/224, various material layers, including the high-k dielectriclayer 226, the conductive layer 228 and the polysilicon layer 230, areformed on the substrate 210 by various deposition technique. Then alithography patterning process is applied to the various material layersto pattern thereof, forming the resistor stack 220 and the gate stacks222/224.

Then source and drain 238 are formed for the nFET 234 by a propertechnique, such as ion implantation. Similarly, source and drain 240 areformed for the pFET 236 by a similar technique. In one embodiment of,the source and drain features (238 and 240) further include light dopedsource/drain (LDD) features aligned with the associated gate stack andheavily doped source/drain (S/D) features aligned with the associatedsidewall spacer 232. In furtherance of the embodiment, taking nFET 234as an example, the LDD features are formed first by ion implantationwith a light doping dose. Thereafter, the spacer 232 is formed bydielectric deposition and plasma etching. Then the heavily doped S/Dfeatures are formed by ion implantation with a heavy doping dose. Thevarious source and drain features of the pFET 236 can be formed in asimilar procedure but with opposite doping type. The sidewall spacer canbe formed in the polysilicon resistor 220 in the process to form thespacers of nFET 234 and the pFET 236.

Still referring to FIG. 2, the method 100 proceeds to step 106 byforming an inter-level dielectric (ILD) layer 242 (also referred to asILD0) on the semiconductor substrate 210, the resistor stack 220 and thegate stacks 222/224. The ILD layer 242 is formed by a suitabletechnique, such as chemical vapor deposition (CVD). For example, a highdensity plasma CVD can be implemented to form the ILD layer 242. The ILDlayer 242 is formed on the substrate to a level above the top surface ofthe resistor stack 220 and the gate stacks 222/224 such that theresistor stack 220 and the gate stacks 222/224 are embedded in.

Referring to FIG. 3, the method 100 proceeds to step 108 by applying achemical mechanical polishing (CMP) process to the ILD layer 242 toreduce the thickness of the ILD layer 242 such that the resistor stack220 and the gate stacks 222/242 are exposed from the top side. Theprocessing conditions and parameters of the CMP process, includingslurry chemical and polishing pressure, can be tuned to partially removeand planarize the ILD layer 242.

Referring to FIG. 4, the method 100 proceeds to step 110 by forming ahard mask layer 244 on the semiconductor structure 200. The hard masklayer 244 includes various openings and is used for the subsequentetching process. In one embodiment, the hard mask layer 244 includesopenings, as illustrated in FIG. 4, to expose the gate stack 224 andpartially expose the resistor stack 220. The formation of the hard masklayer 244 includes deposition and etching. In one example, the hard masklayer 244 includes silicon oxide formed by a CVD, such as plasmaenhanced CVD (PECVD). The hard mask layer 244 may include other suitablematerial such as titanium nitride (TiN) or silicon nitride in variousembodiments. In one example, the TiN hard mask layer can be formed by aphysical vapor deposition (PVD). In another example, the silicon nitridehard mask layer can be formed by a suitable technique, such as PECVD. Inone example, the thickness of the hard mask layer 244 ranges betweenabout 50 angstrom and about 200 angstrom. In another example, the hardmask layer 244 includes a thickness of about 80 angstrom.

Referring to FIG. 5, the method 100 proceeds to step 112 by removing thepolysilicon within the openings of the hard mask layer 244 using anetching process, forming resistor trenches 246 and gate trench 248. Theother regions underlying the hard mask layer 244 are substantiallyprotected from being removed during the etching process. The etchingprocess used to remove the polysilicon within the openings of the hardmask layer 244 may implement suitable dry etching, wet etching orcombinations thereof. In one example, an etching solution includingHNO3, H2O and HF may be used to remove polysilicon. In another example,chlorine (Cl)-based plasma may be used to selectively remove thepolysilicon layer.

Referring to FIG. 6, the method 100 proceeds to step 114 by filling thegate trench 248 and the resistor trenches 246 with p-metal 250 and aconductive material 252, forming a gate electrode for the pFET 236 andvarious electrode features for the resistor stack 220. The polysiliconresistor stack and the electrodes embedded therein constitutes thepassive polysilicon device (or passive device) that can be used as aresistor or a fuse. The passive device is further separately illustratedin FIG. 7 as a top view and is labeled as 253. The passive device 253includes polysilicon feature 230 a and the various electrodes 254. Theelectrodes 254 are formed in the resistor trenches. In the presentembodiment, the electrode 254 includes the p-metal layer 250 and theconductive material layer 252. The p-metal includes a metal-basedconductive material having a work function compatible to the pFET 236.For one example, the p-metal has a work function of about or greaterthan about 5.2 eV. In one embodiment, the p-metal includes titaniumnitride (TiN) or tantalum nitride (TaN). In other embodiments, thep-metal include TiN, TaN, tungsten nitride (WN), titanium aluminum(TiAl), or combinations thereof. The p-metal 250 may include variousmetal-based film as a stack for optimized device performance andprocessing compatibility. The p-metal layer can be formed by a suitableprocess, such as PVD. The conductive material 252 is thereaftersubstantially fills in the gate trench 248 and the resistor trenches246, as illustrated in FIG. 6. The conductive material 252 includestungsten or aluminum according to various embodiments. The method toform the conductive material may include CVD or PVD. Additionally,another CMP process may be applied to remove the excessive p-metal andthe conductive material above the trenches 246/248 and on the ILD layer242.

The hard mask layer 244 in the first region 212 may be thereafterremoved using a procedure, such as a lithography process and an etchingprocess according to one embodiment. Thus, formed passive device 253includes various metal electrodes embedded in the polysilicon feature230 a constructed according to various aspects of the one embodiment.The passive polysilicon device 253 includes the polysilicon feature 230a disposed on the substrate, with a geometry as a rectangle in thisparticular example. The passive device 253 further includes variousconductive columns 254 as the electrodes properly configured andembedded in the polysilicon feature 230 a. The conductive columns 254include the p-metal layer 250 and the conductive material layer 252. Inone embodiment, an electric voltage is applicable between a first subsetof the conductive columns and a second subset of the conductive columnssuch that an electric current can flow in the polysilicon feature 230 afrom the first subset of the conductive columns to the second subset ofthe conductive columns. In this particular example, the electric voltageis applied between the left two and right two of the conductive columns254. The electric current flows from the left two to the right two ofthe conductive columns 254. According to the disclosed structure and themethod of making the passive device 253, the method is compatible to themethod to form a transistor (such as nFET 234 or pFET 236) with a gatestack having high k dielectric and metal electrode, therefore thefabrication cost is reduced. Furthermore, the resistance of thepolysilicon resistor can also be properly controlled and preciselydefined.

Referring to FIG. 8, the method 100 proceeds to step 116 by forming apatterned photoresist layer 256 to cover the passive device (includingresistor stack 220 and the embedded electrodes) in the first region 212.Therefore the formed passive device is protected from subsequent etchingprocess. The patterned photoresist layer 256 can be formed by alithography process known in the art. For example, the lithographyprocess may include spin-on coating, baking, exposure, post-exposurebaking and developing.

Referring to FIG. 9, the method 100 proceeds to step 118 by selectivelyremoving the polysilicon 230 b of the gate stack 222 by an etchingprocess, forming a gate trench 258. Since the selectivity of the etchingprocess to remove the polysilicon, the other conductive materialsexposed substantially remain. Furthermore, since the polysiliconmaterial in the polysilicon resistor is protected by the patternedphotoresist layer 256, it survives after the etching process. Theetching process used to remove the polysilicon may implement suitabledry etching, wet etching or combinations thereof. In one example, anetching solution including HNO3, H2O and HF may be used to removepolysilicon. In another example, chlorine (Cl)-based plasma may be usedto selectively remove the polysilicon layer in the gate stack 222.

Referring to FIG. 10, the method 100 may proceed to step 120 by removingthe patterned photoresist layer 256 using a process, such as wetstripping or plasma ashing, known in the art.

Referring to FIG. 11, the method 100 proceeds to step 122 by filling thegate trench 258 with n-metal 260 and a conductive material 262, forminga gate electrode for the nFET 234. The n-metal includes a metal-basedconductive material having a work function compatible to the nFET 234.For one example, the n-metal has a work function of about or less thanabout 4.2 eV. In one embodiment, the n-metal includes tantalum (Ta). Inother embodiments, the n-metal include titanium aluminum (TiAl),titanium aluminum nitride (TiAlN), or combinations thereof. In otherembodiments, the n-metal include Ta, TiAl, TiAlN, tungsten nitride (WN),titanium nitride (TiN), tantalum nitride (TaN) or combinations thereof.The n-metal 260 may include various metal-based film as a stack foroptimized device performance and processing compatibility. The n-metallayer can be formed by a suitable process, such as PVD. The conductivematerial 262 is thereafter substantially fills in the gate trench 258.The conductive material 262 includes tungsten or aluminum according tovarious embodiments. The method to form the conductive material mayinclude CVD or PVD. Additionally, another CMP process may be applied toremove the excessive n-metal and the conductive material above thetrenches 258 and on the ILD layer 242.

Although not shown, other alternatives and features may present in thepassive device 253 (such as one illustrated FIG. 7) and other processingsteps may present to form various features. In one embodiment, thepassive device 253 can be properly configured and alternatively used asa polysilicon fuse for proper applications. In another embodiment, thepassive device 253 is configured as an array, each being disposed on theshallow trench isolation (STI) 263 and adjacent passive devices beingseparated by active regions 264, as illustrated in FIG. 12 as a top viewof a semiconductor structure 300 having a plurality of passivepolysilicon device 253 constructed according to various aspects of thepresent disclosure in one or more embodiment. The semiconductorstructure 300 is a portion of an integrated circuit having bothplurality of passive polysilicon devices 253 and various field effecttransistors each with a gate stack of high k dielectric and metalelectrode (not shown). The passive polysilicon devices 253 are definedon the STI 263 and interleaved with the active regions 264 to have anuniform structure and optimized device performance. The active region264 can be dummy active region incorporated into the plurality ofpassive polysilicon devices 253. In one example, one or more dummypolysilicon features may be incorporated into the plurality of passivepolysilicon devices 253 to form such an interleaved configuration.According to various embodiments, the passive polysilicon devices 253include n-type or p-type dopants with a doping concentration tuned tohave an intended electric resistance, or alternatively no dopant for ahigher resistance. In another embodiment, the electrodes 254 embedded inthe passive device 253 (such as those in FIG. 7 and not shown here forsimplicity) may have other proper geometries and dimensions. Forexample, the electrodes 254 may have a square or rectangle shape withproper dimensions such that the electric current can uniformly flowand/or the resistance is defined in an optimized mode.

In another embodiment, the p-metal layer 250 and n-metal layer 260 areformed in different order such that n-metal layer 260 is formed firstand the p-metal layer 250 is formed thereafter. In another embodiment,the polysilicon feature is formed with the n-metal layer and istherefore incorporated with the n-metal layer. In another embodiment,the pFET 236 has a strained structure for enhanced carrier mobility andimproved device performance. In furtherance of the embodiment, silicongermanium (SiGe) is formed in the source and drain regions of the pFETto achieve a proper stress effect at step 104. In one example of formingsuch a strained pFET, the silicon substrate within the source and drainregions of the pFET 236 are recessed by one or more etching step. ThenSiGe is epi grown in the recessed regions and heavy doped source anddrain are formed in the epi grown SiGe features. In another example, adummy spacer is formed after the formation of the LDD features. Thedummy spacer is removed after the formation of the SiGe features. Then amain spacer is formed on the sidewalls of the associated gate stack,with a different thickness such that the heavy doped source and drainhave an offset from the SiGe features. For instance, the main spacer isthicker than the dummy spacer such that the heavy doped source and drainare well formed in the SiGe features.

In another embodiment, the nFET 234 has a strained structure forenhanced carrier mobility and improved device performance. Infurtherance of the embodiment, silicon carbide (SiC) is formed in thesource and drain regions of the nFET to achieve a proper stress effectat step 104. The strained nFET can be formed similarly as the strainedpFET is formed. In another embodiment, the n-metal and p-metal layerseach may include other proper metal or metal alloy. In anotherembodiment, the n-metal and p-metal layers each have a multi-layerstructure to have an optimized work function and reduced thresholdvoltage. The gate stacks within the first region and polysilicon stackswithin the second region formed at step 104 may include other materiallayers. For example, an interfacial layer (IL), such as silicon oxide,is formed on the silicon substrate before the formation of the high kdielectric layer. The silicon oxide layer can formed on thesemiconductor substrate by a thermal oxidation or atomic layerdeposition (ALD). The silicon oxide layer can be formed by othersuitable methods such as UV-Ozone Oxidation. The interfacial siliconoxide layer may have a thickness less than 10 A. In another example, acapping layer may be interpose between the high k dielectric layer andthe n-metal (or p-metal) layer. The capping layer in the nFET mayinclude lanthanum oxide (LaO). The capping layer in the pFET may includealuminum oxide (AlO).

Other processing steps may be implemented before, during and/or afterthe formation of the passive device 253, the nFET 234, and the pFET 236.For example, the multilayer interconnection are further formed after thestep 122. The multilayer interconnection includes verticalinterconnects, such as conventional vias or contacts, and horizontalinterconnects, such as metal lines. The various interconnection featuresmay implement various conductive materials including copper, tungstenand silicide. In one example, a damascene process is used to form copperrelated multilayer interconnection structure. In another embodiment,tungsten is used to form tungsten plug in the contact holes.

In one example, the high k dielectric layer can be formed by a suitableprocess such as an atomic layer deposition (ALD). Other methods to formthe high k dielectric material layer include metal organic chemicalvapor deposition (MOCVD), physical vapor deposition (PVD), and molecularbeam epitaxy (MBE). In one embodiment, the high k dielectric materialincludes HfO2. In another embodiment, the high k dielectric materialincludes Al2O3. Alternatively, the high k dielectric material layerincludes metal nitrides, metal silicates or other metal oxides.

In another example, the formation of STI may include etching a trench ina substrate and filling the trench by insulator materials such assilicon oxide, silicon nitride, or silicon oxynitride. The filled trenchmay have a multi-layer structure such as a thermal oxide liner layerwith silicon nitride filling the trench. In one embodiment, the STIstructure may be created using a process sequence such as: growing a padoxide, forming a low pressure chemical vapor deposition (LPCVD) nitridelayer, patterning an STI opening using photoresist and masking, etchinga trench in the substrate, optionally growing a thermal oxide trenchliner to improve the trench interface, filling the trench with CVDoxide, and using chemical mechanical planarization (CMP) to etch back.

The various patterning process may include forming a patternedphotoresist layer formed by a photolithography process. An exemplaryphotolithography process may include processing steps of photoresistcoating, soft baking, mask aligning, exposing, post-exposure baking,developing photoresist and hard baking. The photolithography exposingprocess may also be implemented or replaced by other proper methods suchas maskless photolithography, electron-beam writing, ion-beam writing,thermal lithography, and molecular imprint.

In another embodiment, a hard mask layer may be formed on the gate stacklayers before patterning thereof to form gate stacks and polysiliconstacks at step 104. The hard mask layer is then removed at propersubsequent step or steps. In another embodiment, the patterned hard masklayer includes silicon nitride. As one example of forming the patternedsilicon nitride hard mask, a silicon nitride layer is deposited on thepolysilicon layer by a low pressure chemical vapor deposition (LPCVD)process. The precursor including dichlorosilane (DCS or SiH2Cl2),bis(TertiaryButylAmino)silane (BTBAS or C8H22N2Si) and disilane (DS orSi2H6) is used in the CVD process to form the silicon nitride layer. Thesilicon nitride layer is further patterned using a photolithographyprocess to form a patterned photoresist layer and an etching process toetch the silicon nitride within the openings of the patternedphotoresist layer. Alternatively, other dielectric material may be usedas the patterned hard mask. For example, silicon oxynitride may be usedas the hard mask.

FIG. 13 is a flowchart of another embodiment of a method 400 making asemiconductor structure having a metal gate stack and a passivepolysilicon device constructed according to various aspects of thepresent disclosure. FIG. 14 is a sectional view of the semiconductorstructure, referred to as 450. FIGS. 15 and 16 are sectional views ofthe passive polysilicon device, referred to as 470, constructedaccording to various embodiments. The semiconductor structure 450 andthe method 400 of making the same are collectively described withreference to FIGS. 13 through 16.

The method 400 begins at step 402 by providing a semiconductor substrate210. The semiconductor substrate 210 is similar to the semiconductorsubstrate 210 of FIG. 2. The semiconductor substrate 210 includessilicon. Alternatively, the substrate includes germanium, silicongermanium or other proper semiconductor materials. The semiconductorsubstrate 210 also includes various isolation features such as shallowtrench isolation (STI) formed in the substrate to separate variousdevices. The semiconductor substrate also includes various doped regionssuch as n-well and p-wells. In one embodiment, the semiconductorsubstrate 210 includes a first region 212 and a second region 214. Thefirst region 212 includes a trench isolation feature 216 and the secondregion 214 includes another trench isolation feature 217. The formationof the STI may include etching a trench in a substrate and filling thetrench by insulator materials such as silicon oxide, silicon nitride, orsilicon oxynitride. The filled trench may have a multi-layer structuresuch as a thermal oxide liner layer with silicon nitride filling thetrench. In one embodiment, the STI structure may be created using aprocess sequence such as: growing a pad oxide, forming a low pressurechemical vapor deposition (LPCVD) nitride layer, patterning an STIopening using photoresist and masking, etching a trench in thesubstrate, optionally growing a thermal oxide trench liner to improvethe trench interface, filling the trench with CVD oxide, using chemicalmechanical planarization (CMP) to etch back, and using nitride strippingto leave the STI structure. In another embodiment, the semiconductorsubstrate 210 in the second region 214 includes a p-well 218 and ann-well 219.

The method 400 proceeds to step 404 by forming a polysilicon resistorstack 220 in the first region 212 and gate stacks 222/224 in the secondregion 214. The polysilicon resistor stack (or resistor stack) 220 andthe gate stacks 222/224 are formed in the same processing procedure. Theresistor stack 220 includes a high-k (high dielectric constant)dielectric layer 226 formed on the STI feature 216. The high-kdielectric layer 226 includes a dielectric material having thedielectric constant higher than that of thermal silicon oxide, about3.9. In one example, the high-k dielectric layer 226 includes hafniumoxide (HfO). In various examples, the high-k dielectric layer 226includes metal oxide, metal nitride, or combinations thereof. In oneexample, the high-k dielectric layer 226 includes a thickness rangingbetween about 10 angstrom and about 20 angstrom. In one embodiment, thepolysilicon resistor stack 220 further includes a conductive layer 228disposed on the high-k dielectric layer 226. In one example, theconductive layer 228 includes titanium nitride (TiN). In anotherexample, the thickness of the titanium nitride layer ranges betweenabout 10 angstrom and about 30 angstrom.

The resistor stack 220 further includes a polysilicon layer 230 disposedon the conductive layer 228. The polysilicon layer is non-doped to havea lower resistance or lightly doped to have a proper resistanceaccording to the design. Similarly, the gate stacks 222/224 also includethe high k dielectric layer 226 disposed on the substrate 210 in thesecond region 214. In one embodiment, the gate stacks 222/224 alsoinclude the conductive layer 228 disposed on the high-k dielectric layer226. The gate stacks 222/224 also include the polysilicon layer 230disposed on the conductive layer 228. In one embodiment to form theresistor stack 220 and the gate stacks 222/224, the high k dielectriclayer 226 is first disposed on the semiconductor substrate 210. Theconductive layer 228 is disposed on the high k dielectric layer 226. Thepolysilicon layer 230 is deposited on the conductive layer 228. Then, alithography process and an etching process are applied to the abovematerial layers to pattern thereof, forming the polysilicon resistorstack 220 in the first region 212 and the gate stacks 222/224 in thesecond region 214. In another embodiment, the patterning process mayemploy a hard mask. In this case, a hard mask material is formed on thepolysilicon layer 230. Then a lithography process and an etch processare used to pattern the hard mask layer, forming various openings.Thereafter, an etch process is applied to the various material layers,including the polysilicon layer 230, the conductive layer 228 and thehigh k layer 226, through the openings of the patterned hard mask toform the polysilicon resistor stack 220 and the gate stacks 222/224. Thepolysilicon layer includes various polysilicon features in the resistorstack 220, and the gate stacks 222/224. These polysilicon features, asillustrated in FIG. 14, are referred to as 230 a, 230 b and 230 c,respectively, and are collectively referred to as 230.

In one embodiment, the resistor stack 220 is formed for a passivepolysilicon device that can be used as a resistor or a fuse. The firstgate stack 222 is formed in the second region 214 for an n-typefield-effect transistor (nFET) 234 and the second gate stack 224 isformed in the second region for a p-type field-effect transistor (pFET)236. In one embodiment, the nFET 234 is formed in the p-well 218. Inanother embodiment, the pFET 236 is formed in the n-well 219. The nFET234 further includes various doped regions, such as source/drain 238.Similarly, the pFET 236 further includes various doped regions, such assource/drain 240. In one embodiment, the nFET and pFET includemetal-oxide-semiconductor FETs (MOSFET) such as nMOSFET and pMOSFET.

Alternatively, an interfacial layer (IL), such as silicon oxide, may beinterposed between the high-k dielectric layer 226 and the substrate210. Particularly, the interfacial layer is interposed between the STIfeature 216 and the high-k dielectric layer 226 in the first region 212and is interposed between the substrate 210 and the high-k dielectriclayer 226 in the second region.

The method 400 may proceed to step 406 by performing a light dopingprocess to form light doped drain (LDD) features 452 in the nFET 234.The doping process can be an ion implantation process using n-typedopant, such as phosphorus. A second doping process is also implementedto form LLD features 454 for the pFET 236. The second doping process canalso be an ion implantation process using p-type dopant, such as boron.In one embodiment, spacers 232 are disposed on the sidewalls of theresistor stack 220 and the sidewalls of the gate stacks 222/224 by aprocess including dielectric deposition and etching, known in the art.

The method 400 proceeds to step 408 by applying a heavy doping processto form electrodes embedded in the resistor stack 220. An ionimplantation process is applied to form heavily doped features 458 inthe contact regions of the polysilicon resistor. If the hard mask isused to form the resistor stack 220 and the gate stacks 222/224, thenthe hard mask is removed prior to applying the heavy doping process. Thethus formed heavily doped features 458 in the resistor stack function aselectrodes and are configured for applying electrical voltage to thepassive device during applications. In one embodiment, the electrodes458 may be defined as two sets. The first set of electrodes is formed atone end of the polysilicon feature 230 a of the passive device and thesecond set is formed at the another end of the polysilicon feature 230 aof the passive device, similar to the configuration illustrated in FIG.7 as a top view of the passive device 253 except for that the metalelectrode 254 are replaced by the heavily doped polysilicon electrodes458. Thus formed passive device having the polysilicon resistor stackand the heavily doped polysilicon electrode embedded in the polysiliconresistor stack electrodes. This passive device can be used as apolysilicon resistor or polysilicon fuse according to variousembodiments. In one embodiment, the ion implantation process to form theelectrodes 458 may implement a implantation dose ranging between about1×10¹⁵ and 7×10¹⁵/cm². In another embodiment, an annealing process, suchas rapid thermal annealing (RTA), may be performed thereafter to drivethe doped species deeper. The dopant used to form the electrodes 458 canbe either n-type (such as phosphorus) or p-type (such as boron)according to various embodiments.

In one embodiment, the source and drain 456 are formed for the nFET 234along with the electrodes 458 in the same process. In this case, theelectrodes 458 include n-type dopant. Therefore, no furthermanufacturing cost is added to the semiconductor structure 450. Forexample, a same photomask is used to define various regions includingcontact regions of the passive device and S/D regions of the nFET 234.The pattern defined in the photomask is transferred to a photoresistlayer or a hard mask. The doping process is then applied to thepolysilicon layer 230 through the openings of the photoresist layer orthe hard mask layer to form electrodes in the contact regions of thepassive device and S/D in the S/D regions of the nFET. In anotherembodiment, if the polysilicon gates, such as gate stack 222, remain inthe final product, then the polysilicon gates may also be heavily dopedto enhance the conductivity. In this example, the photomask may furtherincludes openings associated with the polysilicon gates 222. Similarly,source and drain 460 are formed for the pFET 236 by a similar techniqueusing p-type dopant. In an alternative embodiment, the electrodes 458 ofthe passive device are simultaneously formed with the heavily dopedsource/drain 460 of the pFET 236 in the same doping process. In thiscase, the electrodes 458 includes p-type doping species.

As illustrated in FIG. 14, the LDD features are aligned with theassociated gate stack and heavily doped source/drain (S/D) features arealigned with the associated sidewall spacer 232. In furtherance of theembodiment, taking nFET 234 as an example, the LDD features are formedfirst by ion implantation with a light doping dose. Thereafter, thespacer 232 is formed by dielectric deposition and plasma etching. Thenthe heavily doped S/D features are formed by ion implantation with aheavy doping dose. The various source and drain features and thesidewall spacer of the pFET 236 can be formed in a similar procedure butwith opposite doping type. The sidewall spacer of the resistor stack 220may be formed in a process to form the spacers of nFET 234 and/or thepFET 236.

The method 400 may proceed to step 410 by forming silicide 462 on theelectrodes 458 of the passive device. The silicide 462 formed on theelectrodes 458 may further reduce the contact resistance to the passivedevice. The silicide 462 may include nickel silicide. Alternatively, thesilicide 462 may other suitable silicide, such as cobalt silicide,tungsten silicide, tantalum silicide, titanium silicide, platinumsilicide, erbium silicide or palladium silicide. The silicide 462 may beformed by silicidation such as self-aligned silicide (Salicide). In thisprocess, a metal is deposited on silicon or polysilicon. The temperatureof the metal and silicon/polysilicon is raised to a higher level duringand/or after the metal deposition to enhance reaction between Si and themetal in order to form silicide. The unreacted metal is then removed bya process such as etching. The annealing process may be one step ormulti-step annealing depending on metal material and other conditions.For example, after the removal of the unreacted metal, a secondannealing with a temperature higher than that of the first annealing maybe applied to the silicide to turn the silicide into a different phasehaving a lower resistance. In one embodiment, the step 410 to form thesilicide 462 may be implemented right after the heavy doping process ofthe step 409. In furtherance of this embodiment, when the electrodes 458of the passive device are formed with the heavily doped source and drain456 of the nFET 234 in the one process, then the silicide is formed onboth the source and drain regions of the nFET 234 and the electrodes 458of the passive device. The source/drain silicide are not shown in FIG.14 for simplicity.

As one example, the same hard mask used to form the electrodes 458 andthe source and drain 456 can be used during the formation of thesilicide such that the silicide can be formed on the electrodes but noton other regions of the resistor stack 220. Alternatively, when theelectrodes 458 of the passive device are formed with the heavily dopedsource and drain 460 of the pFET 236 in the one process, then thesilicide is formed on both the source and drain regions of the pFET 236and the electrodes 458 of the passive device. The passive device isfurther illustrated in FIGS. 15 and 16 constructed according to variousembodiments. The Passive device 470 includes the polysilicon feature 230a and heavily doped polysilicon electrode 458 embedded in thepolysilicon feature 230 a. In one embodiment, the passive device 470additionally includes the high k dielectric layer 226 underlying thepolysilicon feature 130 a. In another embodiment, the passive device 470further includes the conductive layer 228 interposed between the high kdielectric layer 226 and the polysilicon feature 130 a. and In oneexample illustrated in FIG. 15, the electrodes includes both the heavilydoped polysilicon 458 and the silicide 462. The electrodes areconfigured approximately at the two ends of the polysilicon resistor.

In another example illustrated in FIG. 16, the electrodes includes boththe heavily doped polysilicon 458 and the silicide 462. The electrodesare configured at the two ends of the polysilicon resistor andsubstantially aligned with the edges of the polysilicon feature 230 a ofthe passive device. In another embodiment, the passive device furtherincludes spacers 232 formed on the sidewalls of resistor stack 220. Insome embodiments, the passive polysilicon device 470 thus configured andformed, as illustrated in FIGS. 13-16, can present one or moreadvantages described below. The passive polysilicon device has a higherresistance (for no-doped polysilicon feature 230 a of the passivedevice) or a properly controlled resistance (for lightly dopedpolysilicon feature 230 a of the passive device). The contact resistanceis substantially reduced and an ohm contact is formed due to the heavilydoped polysilicon electrodes and/or silicide formed on the top portionof the electrodes. There is no additional manufacturing cost since noadditional processing steps. The heavy doping process to form theelectrodes can be implemented with the heavy doping process to form thesource/drain of the nFET or the pFET according to various embodiments.In this example, only the layout pattern defined on the photomask, usedfor the nFET (or the pFET) source/drain heavy doping, needs to bemodified to include the openings for the electrodes of the passivedevice.

The method 400 further proceeds to step 412 by forming an interlayerdielectric (ILD) 242 on the semiconductor substrate 210, substantiallycovering the polysilicon passive device and the nFET and pFET. Then achemical mechanical polishing (CMP) process may be applied thereafter topolarize the surface of the ILD 242. Other processing steps may beimplemented to form the functional circuit.

In one embodiment where the polysilicon gate stacks remain in the finaldevice, the corresponding process is referred to as a gate-first processand the thus formed circuit is referred to as a gate-first scheme. Inthe gate-first process, the method 400 proceeds to form interconnectstructures coupled to the passive polysilicon device, nFET and pFET. Forexample, contacts, multilayer metal lines and vias may be formed on thesubstrate to provide proper electrical connection.

In another embodiment where the polysilicon gate stacks are used asdummy gates and are replaced by proper metal gates, the correspondingprocess is referred to as a gate-last process and the thus formedcircuit is referred to as a gate-last scheme. In the gate-last process,the method 400 proceeds to replace the polysilicon gate stacks usingmetal materials. In one example, the polysilicon in the gate stacks ofthe nFET and pFET are removed to form a first gate trench in the nFETand a second gate trench in the pFET. Thereafter, a first metal with afirst work function is formed in the first gate trench and is furtherfilled with a conductive material to form a metal gate for the nFET. Asecond metal with a second work function is formed in the second gatetrench and is further filled with a conductive material to form a metalgate for the pFET. For example, the first work function substantiallyequals to about 4.2 eV and the second work function substantially equalsto about 5.2 eV. Then, formed on the semiconductor substrate are variousinterconnection structures, including contacts, multilayer metal linesand vias. In another example to form metal gates for the nFET and pFET,and the interconnection, the process is substantially similar to theprocessing steps 108 through 122 of FIG. 1 except for that the passivepolysilicon device remains without change through the processing stepsnFET and pFET. For example, at step 112 of FIG. 1, the polysilicon isremoved only from the pFET but not from the polysilicon resistor stack220. The heavily doped polysilicon electrodes remain in the finalpassive polysilicon device.

The semiconductor structures in various embodiments and the methods ofmaking the same may be used in other integrated circuit have a passivepolysilicon device 253 of FIG. 7 or a passive polysilicon device 470 ofFIG. 15 or 16. The passive device 253 includes metal electrodes embeddedtherein. The passive device 470 includes doped polysilicon electrodesembedded therein. For example, the passive device (253 or 470) can beused in an integrated circuit having a high k dielectric layer and metalelectrode, such as strained semiconductor substrate, ahetero-semiconductor device or a stress-free isolation structure. Otherprocessing steps, alternative steps or materials may present in otherembodiments. For example, in the gate-first process to form the heavilydoped electrodes, the polysilicon gates may also be heavily doped in thesame process to reduce the resistance of the gate electrodes. Either ahard mask or a patterned resist layer may be used in the heavily dopingprocess to form the heavily doped electrodes and source/drain. A lightdoping process may be used to dope the polysilicon feature 230 a to tunethe resistance and may be combined with other processing step, such asn-type LDD or p-type LDD, such that the both are formed simultaneouslyin one procedure.

The present disclosure is not limited to applications in which thesemiconductor structure includes a FET (e.g. MOS transistor) and apolysilicon resistor (or polysilicon fuse), and may be extended to otherintegrated circuit having a metal gate stack. For example, thesemiconductor structures may include a dynamic random access memory(DRAM) cell, a single electron transistor (SET), field programmablegate-array (FPGA) and/or other microelectronic devices (collectivelyreferred to herein as microelectronic devices). In another embodiment,the semiconductor structure includes FinFET transistors. Of course,aspects of the present disclosure are also applicable and/or readilyadaptable to other type of transistor, including single-gatetransistors, double-gate transistors and other multiple-gatetransistors, and may be employed in many different applications,including sensor cells, memory cells, logic cells, and others.

Although embodiments of the present disclosure have been described indetail, those skilled in the art should understand that they may makevarious changes, substitutions and alterations herein without departingfrom the spirit and scope of the present disclosure. The semiconductorsubstrate also includes dummy gate stacks for n-type field effecttransistors (nFETs) and p-type FET (pFETs). In one embodiment, the nFETsand pFETs include metal-oxide-semiconductor (MOS) FETS such as nMOSFETsand pMOSFETs. Particularly, the nFETs include and pFETs include dummygate stacks having high k dielectric material and polysilicon material.

The foregoing has outlined features of several embodiments. Thoseskilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions and alterations hereinwithout departing from the spirit and scope of the present disclosure.

What is claimed is:
 1. An integrated circuit, comprising: asemiconductor substrate; and a passive polysilicon device disposed overthe semiconductor substrate, the passive polysilicon device including: apolysilicon feature; and a plurality of electrodes embedded in thepolysilicon feature.
 2. The integrated circuit of claim 1, furthercomprising a high k dielectric layer interposed between thesemiconductor substrate and the passive polysilicon device.
 3. Theintegrated circuit of claim 2, further comprising: a first gate stack ofa n-type field-effect transistor (nFET) formed on the semiconductorsubstrate, the first gate stack including: a first portion of the high kdielectric layer; a first metal layer having a first work function onthe high k dielectric layer; and a first conductive layer on the firstmetal layer; and a second gate stack of a p-type FET (pFET), the secondgate stack including: a second portion of the high k dielectric layer; asecond metal layer having a second work function on the high kdielectric layer; and a second conductive layer on the second metallayer, wherein the second work function is greater than the first workfunction.
 4. The integrated circuit of claim 3, wherein the plurality ofelectrodes comprises: one of the first metal layer and the second metallayer; and one of the first conductive layer and the second conductivelayer.
 5. The integrated circuit of claim 3, wherein the first metallayer include tantalum and the second metal layer includes titaniumnitride.
 6. The integrated circuit of claim 3, wherein each of the firstand second conductive layers each comprises one of aluminum andtungsten.
 7. The integrated circuit of claim 1, wherein the plurality ofelectrodes include heavily doped polysilicon and the polysilicon featureincludes one of non-doped polysilicon and lightly doped polysilicon. 8.The integrated circuit of claim 7, wherein the plurality of electrodesfurther include silicide disposed on the heavily doped polysilicon. 9.The integrated circuit of claim 1, wherein the plurality of electrodescomprises: a first subset of the electrodes configured to be applicablefor a first electric potential; and a second subset of the electrodesconfigured to be applicable for a second electric potential.
 10. Theintegrated circuit of claim 1, wherein the passive polysilicon devicecomprises at least one of a polysilicon resistor and a polysilicon fuse.11. The integrated circuit of claim 1, wherein the passive polysilicondevice is disposed over a shallow trench isolation (STI) disposed in thesemiconductor substrate.
 12. A method of making an integrated circuit,the method comprising: forming a high k dielectric material layer on asemiconductor substrate; forming a polysilicon layer on the high kdielectric material layer; patterning the polysilicon layer to form apolysilicon feature for a passive device; and forming a plurality ofelectrodes embedded in the polysilicon feature.
 13. The method of claim12, wherein the forming of the plurality of electrodes includes applyingan ion implantation process to the polysilicon feature to form theplurality of electrodes.
 14. The method of claim 12, wherein the formingof the plurality of electrodes includes forming a patterned mask havingopenings defining contact regions on the polysilicon feature; andthereafter applying an ion implantation process to the polysiliconfeature within the openings.
 15. The method of claim 12, wherein theforming of the plurality of electrodes include forming an inter-leveldielectric (ILD) layer on the semiconductor substrate; performing afirst chemical mechanical polishing (CMP) process to etch back the ILDlayer; removing a portion of the polysilicon feature thereby forming aplurality of column trenches in the passive device; and forming a firstmetal layer in the column trenches, wherein the first metal layer has afirst work function.
 16. The method of claim 15, wherein the patterningof the polysilicon layer further includes forming a first dummy gate fora first field-effect transistor (FET) and a second dummy gate for asecond FET; the removing further includes removing the first dummy gatefrom the first FET thereby forming a first gate trench in the first FET;and the forming further includes forming the first metal layer in thefirst gate trench.
 17. The method of claim 16, further comprising:removing the second dummy gate from the second FET thereby forming asecond gate trench in the second FET; and forming a second metal layerin the second gate trench, wherein the second metal layer has a secondwork function different from the first work function.
 18. The method ofclaim 16, wherein the one of the first FET and the second FET is ann-type FET and the other one of the first FET and the second FET is ap-type FET.
 19. The method of claim 16, further comprising performing asecond CMP process to remove excessive portion of the first metal layer.20. The method of claim 19, further comprising forming a conductivelayer on the first metal layer prior to the performing of the second CMPprocess.
 21. A method of making an integrated circuit, the methodcomprising: forming a high k dielectric material layer on asemiconductor substrate; forming a polysilicon layer on the high kdielectric material layer; patterning the polysilicon layer to form afirst dummy gate for a first field-effect transistor (FET), a seconddummy gate for a second FET, and a polysilicon feature for a passivedevice; forming an inter-level dielectric (ILD) layer on thesemiconductor substrate; performing a first chemical mechanicalpolishing (CMP) process to the ILD layer; removing a portion of thepolysilicon feature from the passive device and the first dummy gatefrom the first FET, resulting in a plurality of column trenches in thepassive device and a first gate trench in the first FET; forming a firstmetal layer in the column trenches and the first gate trench, whereinthe first metal layer has a first work function; forming a firstconductive layer on the first metal layer; and performing a second CMPprocess to remove excessive portion of the first metal layer and thefirst conductive layer above a top surface of the ILD layer.
 22. Themethod of claim 21, further comprising: removing the second dummy gatefrom the second FET, resulting in a second gate trench in the secondFET; forming a second metal layer in the second gate trench, wherein thesecond metal layer has a second work function different from the firstwork function; forming a second conductive layer on the second metallayer; and performing a third CMP process to remove excessive portion ofthe second metal layer and the second conductive layer above the topsurface of the ILD layer.
 23. The method of claim 22 further comprisingforming a patterned photoresist layer to cover the passive device beforethe removing of the second dummy gate from the second FET.
 24. A methodof making an integrated circuit, the method comprising: forming a high kdielectric material layer on a semiconductor substrate; forming apolysilicon layer on the high k dielectric material layer; patterningthe polysilicon layer to form a first gate for a first field-effecttransistor (FET), a second gate for a second FET, and a polysiliconfeature for a passive device; forming, on the semiconductor substrate, apatterned mask having a first set of openings defining contact regionson the polysilicon feature of the passive device, and a second set ofopenings defining source/drain regions in the first FET; and applying afirst ion implantation to the semiconductor substrate to form:electrodes embedded in the polysilicon feature within the first set ofopenings; and source and drain of the first FET within the second set ofopenings.
 25. The method of claim 24, further comprising applying asilicidation process to form silicide on the electrodes.
 26. The methodof claim 24, further comprising a second ion implantation to form sourceand drain of the second FET, wherein the first FET is one of n-type andp-type, and the second FET is the other one of n-type and p-type. 27.The method of claim 24, wherein the patterned mask includes one of apatterned resist layer and a patterned hard mask.
 28. The method ofclaim 24, further comprising replacing the first gate using a firstmetal having a first work function and replacing the second gate using asecond metal having a second work function.